Fortune never comes with an parallel and misfortune never comes alone.

解释 ( paraphrase )
指幸运事不会连续到来，祸事却会接踵而至。
这条谚语告诉我们：无论遇到顺心之事祸事不顺心之事，都要平复好自己的感情，过于兴奋会使我们错过下个幸运的事情，过于沉迷于消极的情绪会使我们用这种情绪看待所有事情，祸事便是由此而来。

This proverb tells us that the feeling that misfortunes never seem to be isolated could be a product of our own negative thoughts, brought about by a single misfortune, or the misfortune may trigger a series of connected misfortunes.

造句 ( example )
真是“福无双至，祸不单行”啊，最近咱们家真是祸事连连啊。